1. Field of the Invention
The petroleum industry has long been interested in the production of "synthetic" liquid fuels from non-petroleum solid fossil fuel sources. It is hoped that economic non-petroleum sources of liquid fuel will help the petroleum industry to meet growing energy requirements and decrease dependence on foreign supplies.
Coal is the most readily available and most abundant solid fossil fuel, others being tar sands and oil shale. The United States is particularly richly endowed with well distributed coal resources. Additionally, in the conversion of coal to synthetic fuels, it is possible to obtain liquid yields of about three to four barrels per ton of dry coal, or about four times the liquid yield/ton of other solid fossil fuels such as tar sands or shale, because these resources contain a much higher proportion of mineral matter.
Despite the continued interest and efforts of the petroleum industry in coal hydroconversion technology, further improvements are necessary before it can reach full economic status. Maximizing the yield of coal liquids is important to the economics of coal hydroconversion.
The present invention relates to an improved process for converting coal to liquid hydrocarbon products in a catalytic hydroconversion process. The improvement relates to a coal pretreatment stage comprising subjecting the coal to aqueous carbon monoxide under specific pressure and temperature conditions. Such pretreatment enhances solubility in the subsequent coal extraction stage. The reactivity of the coal extract in the subsequent hydroconversion stage is advantageously high.
2. Description of the Prior Art
The known processes for producing liquid fuels from coal can be grouped into four broad categories: direct hydrogenation, donor solvent hydrogenation, Fischer-Tropsch synthesis (via gasification), and pyrolysis (see Kirk Othmer - Fuels).
The direct hydrogenation of coal in the presence of solvent and catalyst was first developed in Germany prior to World War II. In such a process, a slurry of coal in a suitable solvent was reacted in the presence of molecular hydrogen at an elevated temperature and pressure.
A number of previous co-assigned patents disclose coal liquefaction processes utilizing hydroconversion catalysts which are micron-sized particles comprised of a metal sulfide in a carbonaceous matrix. These catalysts are generally formed from certain soluble or highly dispersed organometallic or inorganic compounds or precursors. These precursors are converted into catalyst particles by heating in the presence of a hydrogen-containing gas. The catalyst particles are highly dispersed in the feed being treated during hydroconversion. Among the various patents in this area are U.S. Pat. No. 4,077,867; U.S. Pat. No. 4,094,765; U.S. Pat. No. 4,149,959; U.S. Pat. No. 4,298,454; and U.S. Pat. No. 4,793,916. Other patents disclose catalysts similar to the above except that the catalytically active metal compound is supported on finely divided particles of solid metals and metal alloys, for example as disclosed in U.S. Pat. Nos. 4,295,995 and 4,357,229.
The conversion of coal in the presence of high temperature steam and carbon monoxide is well known, dating back to Fischer and Schrader in 1921 (F. Fisher & H. Schrader, Bennst. Chem., 2, 257, 1921). Several hydroconversion processes, including the U.S. Bureau of Mines COSTEAM process (H. R. Appell, E. C. Moroni, R. D. Miller, Energy Sources, 3, 163, 1971), have been developed based on using steam/carbon monoxide or steam/syngas at 750-850.degree. F. in a primary conversion step. In contrast, the present invention is directed to the use of an aqueous carbon monoxide environment for pretreatment of coal before a subsequent primary conversion step.
One of the problems encountered in certain catalyzed coal hydroconversion processes is the separation of slurried catalyst from solid by-products, such as undissolved organic coal and ash. Such solid materials are typically dispersed throughout the reaction mixture during the hydroconversion operation, and are thus present in the coal liquid recovered after hydroconversion. Such solid materials are present in the coal liquids in a finely divided, particulate state, and are typically separated from the coal liquid products by distillation.
Another problem inherent in coal hydroconversion processes has been the requirement for large amounts of hydrogen. It has been suggested that this problem of hydrogen consumption could be reduced by converting only a relatively small fraction of the coal, which fraction is rich in hydrogen. However, to be economical, there is a need for a process which converts a relatively large fraction of the coal to valuable liquid hydrocarbon products. The present process, while not necessarily reducing the requirement for hydrogen, allows coal to be taken to a higher conversion level. Hydrogen utilization is therefore more efficient. For a given amount of liquid products less gas is produced, resulting in a better liquid to gas selectivity.
An object of the present invention is to provide a novel process for the hydroconversion of coal in order to produce valuable liquid hydrocarbonaceous products.
A further object of the present invention is to provide an improved process for producing liquid hydrocarbonaceous products from coal by utilizing a pretreatment step wherein the coal is subjected to reaction with aqueous carbon monoxide.
A still further object of the present invention is to pretreat coal in a specific temperature range to enhance extraction and generate a more reactive coal material for hydroconversion, thereby obtaining more product, with better liquid to gas selectivity.
Another object of the present invention is to improve the utilization efficiency of molecular hydrogen, in the transformation of coal to valuable liquids, by sending a more hydrogenated fraction of the coal to hydroconversion, as well as effecting better liquid-to-gas selectivity.
Another object of the present invention is to increase the thermal efficiency of a coal hydroconversion plant by providing a more efficient coal dewatering and coal partial oxidation operation.
Still another object of the present invention is to liquefy coal by a process comprising in sequence a pretreatment stage, an extraction stage (ex-situ or in-situ), and a catalytic hydroconversion stage.
Additional advantages of the present coal hydroconversion process will become apparent in the following description.